UMass Amherst KIN 460 - 13-Neural Control of Posture and Balance (46 pages)

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13-Neural Control of Posture and Balance



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13-Neural Control of Posture and Balance

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Pages:
46
School:
University of Massachusetts Amherst
Course:
Kin 460 - Motor Control
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What determines stable upright posture Questions What are the vestibular system contributions to posture What are the somatosensory contributions to posture What are the visual contributions to posture What is the role of the brainstem in postural control How do the muscle respond to postural disturbances POSTURAL CONTROL POSTURAL ORIENTATION posture POSTURAL STABILITY equilibrium balance Alignment of body segments Body s orientation in the environment Control of centre of mass COM relative to base of support BOS Postural muscles and the ideal postural configuration Postural stability If an object falls over or not is entirely a matter of the vertical projection of its center of gravity relative to its supports If this line of projection X lies within the area defined by the points of contact with the ground the support area the object does not fall left center If X lies outside this critical area then the system is unstable and any further tilting will cause an ever increasing rotational force that will make the object fall right Red arrows represent the differences of pressure at different points of support The man clearly has an excellent upright posture but is equally clearly about to experience a postural disaster because the vertical projection of his center of gravity lies outside his region of contact with his support What determines stable upright posture Questions What are the vestibular system contributions to posture What are the somatosensory contributions to posture What are the visual contributions to posture What is the role of the brainstem in postural control How do the muscle respond to postural disturbances Vestibular contribution to Control of upright stance Otolith receptors Static response and population coding head orientation Phasic response linear acceleration deceleration Semicircular canals Phasic response angular acceleration deceleration Left versus right response direction of head movement VOR coordinates eye head movements Otolith receptors detect head orientation and linear accelerations Axis of mechanical sensitivity of each hair cell in the utricle is oriented toward the striola Results in variation in axes arrows in population of hair cells Therefore tilt in any direction depolarizes some cells hyperpolarizes some cells and has no effect on some cells Utricle and sacculus combined track linear forces acting on the head in any direction Otolith receptors are stimulated by head orientation and linear acceleration When otolith organ is tilted force of gravity pulling on the otoconia causes them to move relative to the macular surface because otolithic membrane deforms when head accelerates utricle and hair cell bodies accelerate but otoconia tend to stay where they are so are displaced relative to the macula Static contributions Tonic postural vestibular reflexes If an animal is suspended in the air and its head and body tilted nose downwards extension of the front legs and retraction of the rear can be observed Vestibular system responsible otoliths Semicircular canals Sensory receptors hair cells in the ampulla Sense head rotations Receptors in in the ampula are stimulated by angular accelerations Paired canals work together to signal head movement Because of inertia rotation of the head in CCW direction causes endolymph to move clockwise with respect to the canals Causes depolarization on the left In the right canal hair cells are hyperpolarized and afferent firing decreases Functional organization of the semicircular canals Paired canals work together to signal head movement Perpendicular canal arrangement for movement coding Receptors are stimulated by angular accelerations of the head about different axes of rotation Horizontal canal receptors head rotation about vertical axis Anterior posterior canals two perpendicular axes that lie in horizontal plane Vestibulo ocular reflex VOR Compensates for head movement Vestibular system signals how fast head is moving and oculomotor system uses this information to stabilize the eyes Head movement to left eyes deviate to right vice versa Often need to suppress VOR when it is not needed Dynamic contributions Vestibular nystagmus Rotating chair Head motion results in smooth counter rotation of the eyes in opposite direction to head motion slow phase interrupted at more or less regular intervals by a quick flick in the other direction quick phase giving rise to a sawtooth like eye movement Initiated by semicircular canals Connections underlying the vestibulo ocular reflex 1 Head turns left 2 Eyes turn right 3 Activate left medial rectus and right lateral rectus Galvanic Vestibular Stimulation GVS Application of weak electrical stimulation to mastoid bones that carries through the head and interferes with vestibular system Leads to postural sway responses When seated leads to illusion of moving GVS elicits sway in the direction opposite to the negative electrode the side the person perceives they are tilting to This response is automatic and involuntary https youtu be Px6sOkjj ZE t 2m35s What determines stable upright posture Questions What are the vestibular system contributions to posture What are the somatosensory contributions to posture What are the visual contributions to posture What is the role of the brainstem in postural control How do the muscle respond to postural disturbances Somatosensory contributions to postural control Pressure receptors in the feet pressure at different points of contact Extensor thrust on pushing up on the sole of one of its feet the animal responds by extending the corresponding limb center of mass Loss of postural stability associated with increased Center of pressure CoP or Center of Mass CoM excursion or variability Announcements November 30 Team project 4 Traumatic Brain Injury TBI Concussion Case Studies Readings Mild Traumatic brain injury Neuroimaging of Sports related concussion Mendez Consensus statement on concussion in sport the 4th International Conference on Concussion in Sport held in Zurich November 2012 McCrory et al A Brief Vestibular Ocular Motor Screening VOMS Assessment to Evaluate Concussions Mucha et al VOMS and GCS score sheets insturctions Review center of mass Loss of postural stability associated with increased Center of pressure CoP or Center of Mass CoM excursion or variability Review Vestibular and somatosensory contributions to posture control Patients with vestibular impairment make normal postural muscle reactions to platform sliding


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